The present invention relates to an apparatus for obtaining a sample of liquid from near the bottom of a liquid-filled container and more particularly relates to a liquid sampling gauge apparatus for obtaining a sample from a layer of water or water-oil emulsion underlying crude oil in a cargo tank and for determining the depth of said layer.
In many instances, cargo oil, e.g. crude or refined oil which is loaded onto tanker ships, is contaminated with substantial amounts of water. Because of differences in certain physical properties such as density and specific gravity, oil forms the top layer. One cannot readily ascertain how much oil or water is in a cargo tanker by mere visual inspection, and it can be easily understood that if payment for the purchased oil is based on the measured volume of total liquid in the cargo tanks, the final cost to a buyer may be substantially inflated since he pays for the useless water at cargo oil prices. In many actual cargo oil purchasing situations existing today, the buyer is allowed to deduct the volume of water in the cargo tanks only if he can prove that it was pumped aboard with the cargo oil and was not already present in the tanks before the cargo oil was loaded.
Since water inadvertently left in the tanks (e.g. seawater used for ballast, etc.), will have a substantially different composition from the water loaded with the cargo oil (e.g. formation water produced with the crude oil or condensate from steam used in processing the produced oil, etc.), the origin of the water in question can readily be established if a sample of the water can be obtained from a cargo tank for analysis after it has been loaded.
Further, by knowing the origin of any water in a loaded cargo tank, steps can be taken to alleviate the contamination problem. That is, if the water is being pumped in with the cargo oil, further processing of the cargo oil can be undertaken to remove the water before loading, or if the water is originally present in the tank, further steps can be taken aboard the ship to remove such water before loading. Also, it is necessary to determine the depth of the water layer so that the volume of water in the filled tank can be deducted from the total volume of liquid in the tank. Therefore, it can readily be seen that an important need exists both for obtaining a sample of any water that may be present in a cargo tank after it has otherwise been loaded with crude or refined oil, and for determining the depth of said water in the tank.
Several sampling devices have been proposed for this purpose. For example, in copending U.S. patent application Ser. No. 937,293, filed Aug. 2, 1978, a sampling device is disclosed which is comprised of a flexible conduit, normally closed at its upper end. The conduit is lowered with a sounding bob into a filled cargo tank until the lower end of the conduit is positioned within the water layer on the bottom of the tank. The conduit is then opened and suction is applied to withdraw a sample of the water through the conduit. Another sampling device of this general type is disclosed in copending U.S. patent application Ser. No. 68,995, filed Aug. 23, 1979. This sampling device is comprised of a cylinder having a piston positioned therein. The cylinder is lowered on a line into the liquid to be sampled. The line is then lifted to raise the piston relative to the cylinder to draw the liquid to be sampled into the cylinder.
Other known sampling devices for obtaining samples from a liquid-filled tank are disclosed in U.S. Pat. Nos. 1,511,591; 2,593,830; and 3,129,513; all of which disclose lowering a device comprising either an open tube or cylinder into a liquid-filled tank and then closing a valve at the lower end of the tube or cylinder once the device is in place to trap a sample of fluid therein for retrieval.